1. Field of the Invention
The present invention relates to a superconducting current limiting apparatus which employs a superconductor, and, more particularly, to a superconducting current limiting apparatus which is capable of limiting the flow of short circuit current which an electric power system delivers when a short circuit is established so as to lower the level at which the components of the power system are required to withstand short-circuit currents. Furthermore, the present invention relates to a superconducting current limiting apparatus which employs as a current limiting element a superconductor which exhibits superconductivity at a temperature above the boiling point of liquid nitrogen.
2. Description of the Related Art
With the steady increase in the demand for electric power which we have seen in recent years, the level of short-circuit current which a power transmission or transformation system delivers when a short circuit is established in it is gradually rising. This rise in the voltage of short-circuit current represents a threat to the components of the electric power system, as well as to circuit breakers adapted to interrupt the flow of this short-circuit current, and, hence, there has been a demand for the development of a current limiting apparatus which is capable of limiting the flow of short-circuit current. This is in line with the demand for a current limiting apparatus which is capable of limiting current flow immediately so that currently used components of a power supply or transmission lines can continue to be used without replacement and also so that manufactured components or transmission lines can be designed to have a low short-circuit current capacity.
Further, the level of short-circuit current which flows in household distribution systems is also rising, and a demand has arisen for a high quality power supply which does not suffer from instantaneous power failures, just like the one supplied to electronic calculators that have grown in popularity. In household distribution systems, a protective device for limiting the current such as a fuse may be used in a case wherein there is no need for specifying any abnormally connected points in a circuit and providing for selective breaking of the circuit. However, such a fuse must be replaced to restore power each time a short-circuit occurs, even when instantaneous short-circuiting caused by such events as lightning is the cause. This means that the power supply is frequently interrupted for a long period of time, and is therefore undesirable from the viewpoint of ensuring a stable power supply. Such a prolonged interruption of service is not permissible in a power supply system. No currently available current limiting apparatus is capable of limiting current flow and of speedily restoring power to normal when the detrimental excess current has been eliminated, and hence, current limiting apparatuses have not been used in power supply systems.
Research has been conducted on current limiting apparatuses in the United States of America for many years, and various reports have been issued so far. Among them is the EL-276-SR Special Report for the Electric Research Institute (EPRI): Symposium Proceedings, "New Concepts in Fault Current limiters and Power Circuit Breakers". The basic requirements of a current limiting apparatus are to increase impedance at the time of an abnormal connection between two points of a circuit and to limit the flow of a short-circuit current. Various current limiting apparatuses which fulfill this function have been proposed. However, only current limiting apparatuses with a fuse are practical from the viewpoint of stabililty of operation and the ability to restore power supply. This type of current limiting apparatus is disclosed, for example, in the specifications of Japanese Patent Laid-Open Nos. 56-57236 and 47-40146. These current limiting apparatuses with a fuse utilize changes in the state of metal of alkali type. However, this type of current limiting apparatus has the disadvantage that it is difficult to develop a large voltage drop, and use thereof is therefore limited to limitation of low voltages of up to several thousands volts.
Research on current limiting apparatuses which utilize superconductors have also been conducted. However, a conventional current limiting apparatus employing liquid helium involves the problem that it is difficult to radiate the heat generated in a normal conducting state, and these have not yet been put into practical use.
Recently, high-temperature superconducting materials have been developed rapidly, and materials which exhibit superconductivity at a temperature beyond the liquid nitrogen boiling point of 77 K have been developed. Such superconductive materials include a ceramic formed of Y-Ba-Cu-O compounds.
Superconducting current limiting apparatuses limit the flow of a short-circuit current utilizing an increase in an electrical resistance which occurs after the superconducting material has been rendered to a normal conducting state (after it has quenched). In order to suppress the rise in a temperature which occurs when the material quenches, superconducting materials which are employed in the current limiting apparstus must have a relatively low current density and a large cross-sectional area. Further, it must be elongated in order to obtain a large resistance. Use of such a superconducting material may allow a resultant current limiting apparatus to be used even when it generates a large amount of heat after the current density of the material has exceeded a critical current density Jc and the superconducting material has been rendered to a normal conducting state. This is because, when compared with a conventional current limiting apparatus employing a superconducting material which exhibits a superconductivity at a temperature of the boiling point of liquid helium, a current limiting apparatus employing the above-described superconducting material can be used at a higher temperature and therefore has a higher specific heat and because it has a critical temperature higher than the boiling point of liquid nitrogen, and therefore has a larger heat capacity.
Current limiting apparatuses which employ a superconducting wire so as to limit the flow of a short-circuit current utilizing a rise in the electrical resistance which occurs when the superconducting wire quenches have been proposed.
In such an apparatus, the superconducting wire produces Joule heat once it has been rendered to a normal state, and the heated superconducting wire must be therefore cooled by a refrigerant to restore a superconducting state and the refrigerant must be poured to a predetermined level. These operations normally take a time ranging between several tens minutes to a few hours, and the line cannot be used during that time. Normally, in a power supply system, power must be restored 0.3 to 0.5 seconds after the current flow has been interrupted so as to avoid occurrence of a power failure. In the case wherein a relay system for specifying abnormally connected points and for interrupting the flow of a current in the required portion of a circuit is used, since it takes about 2 seconds for the relay system to start operation, the short-circuit current is made to flow during that time (short-time duty). Thereafter, the superconducting wire must be returned to a superconducting state from a normal state in 0.2 seconds since the power must be restored (speedy closing of a circuit) 0.2 seconds after the flow of the short-circuit current has been interrupted. In other words, the superconducting wire which has been heated by Joule heat as a resistor for 2 seconds must be cooled to a superconducting transition temperature (a critical temperature) 0.2 seconds after the flow of the current has been interrupted.
Thus, in order to maintain a superconducting current limiting element to a superconducting state, provision of an effective cooling means for maintaining the temperature of the element below a superconducting transition temperature is essential.